(3588) Complementכִּיkiy{kee}A primitive particle (the full form of the prepositional prefix) indicating causal relations of all kinds, antecedent or consequent; (by implication) very widely used as a relative conjugation or adverb; often largely modified by other particles annexed.

(0853) Complementאֵת'eth{ayth}Apparently contracted from H0226 in the demonstrative sense of entity; properly self (but generally used to point out more definitely the object of a verb or preposition, even or namely).

{3117} Primeיוֹםyowm{yome}From an unused root meaning to be hot; a day (as the warm hours), whether literally (from sunrise to sunset, or from one sunset to the next), or figuratively (a space of time defined by an associated term), (often used adverbially).

(3588) Complementכִּיkiy{kee}A primitive particle (the full form of the prepositional prefix) indicating causal relations of all kinds, antecedent or consequent; (by implication) very widely used as a relative conjugation or adverb; often largely modified by other particles annexed.

{1892} Primeהֶבֶלhebel{heh'-bel}From H1891; emptiness or vanity; figuratively something transitory and unsatisfactory; often used as an adverb.

Ecclesiastes 11:8

_ _ But while man thankfully enjoys life, “let him remember” it will not last for ever. The “many days of darkness,” that is, the unseen world (Job 10:21, Job 10:22; Psalms 88:12), also days of “evil” in this world (Ecclesiastes 11:2), are coming; therefore sow the good seed while life and good days last, which are not too long for accomplishing life’s duties.

_ _ All that cometh  that is, All that followeth in the evil and dark days is vain, as far as work for God is concerned (Ecclesiastes 9:10).

Ecclesiastes 11:8

Rejoice  Enjoy all the comforts, and escape all the embitterments of human life, all his days. Darkness  Of death, or of the state of the dead. Many  Far more than the days of this short life. All  All things which befall any man belonging only to this life, are but vain, because they are short and transitory.

Ecclesiastes 11:8

But if a man shall live many years, [and] rejoice in them all; yet let him remember the days of (g) darkness; for they shall be many. All that cometh [is] vanity.

(g) That is, of affliction and trouble.

if a man,Ecclesiastes 6:6 Yea, though he live a thousand years twice [told], yet hath he seen no good: do not all go to one place?Ecclesiastes 8:12 Though a sinner do evil an hundred times, and his [days] be prolonged, yet surely I know that it shall be well with them that fear God, which fear before him:

rejoice:

Ecclesiastes 3:12-13 I know that [there is] no good in them, but for [a man] to rejoice, and to do good in his life. ... And also that every man should eat and drink, and enjoy the good of all his labour, it [is] the gift of God.Ecclesiastes 5:18-20 Behold [that] which I have seen: [it is] good and comely [for one] to eat and to drink, and to enjoy the good of all his labour that he taketh under the sun all the days of his life, which God giveth him: for it [is] his portion. ... For he shall not much remember the days of his life; because God answereth [him] in the joy of his heart.Ecclesiastes 8:15Then I commended mirth, because a man hath no better thing under the sun, than to eat, and to drink, and to be merry: for that shall abide with him of his labour the days of his life, which God giveth him under the sun.

yet:

Ecclesiastes 7:14 In the day of prosperity be joyful, but in the day of adversity consider: God also hath set the one over against the other, to the end that man should find nothing after him.Ecclesiastes 12:1-5Remember now thy Creator in the days of thy youth, while the evil days come not, nor the years draw nigh, when thou shalt say, I have no pleasure in them; ... Also [when] they shall be afraid of [that which is] high, and fears [shall be] in the way, and the almond tree shall flourish, and the grasshopper shall be a burden, and desire shall fail: because man goeth to his long home, and the mourners go about the streets:Deuteronomy 32:29O that they were wise, [that] they understood this, [that] they would consider their latter end!Job 10:22 A land of darkness, as darkness [itself; and] of the shadow of death, without any order, and [where] the light [is] as darkness.Job 14:10 But man dieth, and wasteth away: yea, man giveth up the ghost, and where [is] he?Job 15:23 He wandereth abroad for bread, [saying], Where [is it]? he knoweth that the day of darkness is ready at his hand.Job 18:18He shall be driven from light into darkness, and chased out of the world.Jeremiah 13:16Give glory to the LORD your God, before he cause darkness, and before your feet stumble upon the dark mountains, and, while ye look for light, he turn it into the shadow of death, [and] make [it] gross darkness.Joel 2:2A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread upon the mountains: a great people and a strong; there hath not been ever the like, neither shall be any more after it, [even] to the years of many generations.Matthew 22:13 Then said the king to the servants, Bind him hand and foot, and take him away, and cast [him] into outer darkness; there shall be weeping and gnashing of teeth.John 12:35 Then Jesus said unto them, Yet a little while is the light with you. Walk while ye have the light, lest darkness come upon you: for he that walketh in darkness knoweth not whither he goeth.Jude 1:18 How that they told you there should be mockers in the last time, who should walk after their own ungodly lusts.

All that:

Ecclesiastes 2:1-11 I said in mine heart, Go to now, I will prove thee with mirth, therefore enjoy pleasure: and, behold, this also [is] vanity. ... Then I looked on all the works that my hands had wrought, and on the labour that I had laboured to do: and, behold, all [was] vanity and vexation of spirit, and [there was] no profit under the sun.Ecclesiastes 2:15 Then said I in my heart, As it happeneth to the fool, so it happeneth even to me; and why was I then more wise? Then I said in my heart, that this also [is] vanity.Ecclesiastes 2:17 Therefore I hated life; because the work that is wrought under the sun [is] grievous unto me: for all [is] vanity and vexation of spirit.Ecclesiastes 2:19 And who knoweth whether he shall be a wise [man] or a fool? yet shall he have rule over all my labour wherein I have laboured, and wherein I have shewed myself wise under the sun. This [is] also vanity.Ecclesiastes 2:21-23 For there is a man whose labour [is] in wisdom, and in knowledge, and in equity; yet to a man that hath not laboured therein shall he leave it [for] his portion. This also [is] vanity and a great evil. ... For all his days [are] sorrows, and his travail grief; yea, his heart taketh not rest in the night. This is also vanity.Ecclesiastes 2:26 For [God] giveth to a man that [is] good in his sightwisdom, and knowledge, and joy: but to the sinner he giveth travail, to gather and to heap up, that he may give to [him that is] good before God. This also [is] vanity and vexation of spirit.Ecclesiastes 4:8 There is one [alone], and [there is] not a second; yea, he hath neither child nor brother: yet [is there] no end of all his labour; neither is his eye satisfied with riches; neither [saith he], For whom do I labour, and bereave my soul of good? This [is] also vanity, yea, it [is] a sore travail.Ecclesiastes 4:16 [There is] no end of all the people, [even] of all that have been before them: they also that come after shall not rejoice in him. Surely this also [is] vanity and vexation of spirit.Ecclesiastes 5:15-16 As he came forth of his mother's womb, naked shall he return to go as he came, and shall take nothing of his labour, which he may carry away in his hand. ... And this also [is] a sore evil, [that] in all points as he came, so shall he go: and what profit hath he that hath laboured for the wind?Ecclesiastes 6:11 Seeing there be many things that increase vanity, what [is] man the better?